When the contained value is not trivially copy (or move) constructible
the union's copy (or move) constructor will be deleted, and so the
_Optional_payload delegating constructors are invalid. G++ fails to
diagnose this because it incorrectly performs copy elision in the
delegating constructors. Clang does diagnose it (llvm.org/PR40245).

The solution is to avoid performing any copy (or move) when the
contained value's copy (or move) constructor isn't trivial. Instead the
contained value can be constructed by calling _M_construct. This is OK,
because the relevant constructor doesn't need to be constexpr when the
contained value isn't trivially copy (or move) constructible.

Additionally, this patch removes a lot of code duplication in the
_Optional_payload partial specializations and the _Optional_base partial
specialization, by hoisting it into common base classes.

The Python pretty printer for std::optional needs to be adjusted to
support the new layout. Retain support for the old layout, and add a
test to verify that the support still works.

	PR libstdc++/87855
	* include/std/optional (_Optional_payload_base): New class template
	for common code hoisted from _Optional_payload specializations. Use
	a template for the union, to allow a partial specialization for
	types with non-trivial destructors. Add constructors for in-place
	initialization to the union.
	(_Optional_payload(bool, const _Optional_payload&)): Use _M_construct
	to perform non-trivial copy construction, instead of relying on
	non-standard copy elision in a delegating constructor.
	(_Optional_payload(bool, _Optional_payload&&)): Likewise for
	non-trivial move construction.
	(_Optional_payload): Derive from _Optional_payload_base and use it
	for everything except the non-trivial assignment operators, which are
	defined as needed.
	(_Optional_payload<false, C, M>): Derive from the specialization
	_Optional_payload<true, false, false> and add a destructor.
	(_Optional_base_impl::_M_destruct, _Optional_base_impl::_M_reset):
	Forward to corresponding members of _Optional_payload.
	(_Optional_base_impl::_M_is_engaged, _Optional_base_impl::_M_get):
	Hoist common members from _Optional_base.
	(_Optional_base): Make all members and base class public.
	(_Optional_base::_M_get, _Optional_base::_M_is_engaged): Move to
	_Optional_base_impl.
	* python/libstdcxx/v6/printers.py (StdExpOptionalPrinter): Add
	support for new std::optional layout.
	* testsuite/libstdc++-prettyprinters/compat.cc: New test.

From-SVN: r267742